Conventional hot-air circulation-type heating furnaces include, for example, one such as shown in FIG. 9 (Japanese Patent Laid-Open No. 2002-173708). This heating furnace has a furnace body 101 made of fire-resistive material and a heating-target-accommodating casing 102 in the form of a cylinder opened at its upper and lower ends and arranged coaxially with the furnace body 101. In this heating furnace, hot air generated by a burner 105 provided on a furnace bottom portion is forcibly circulated as a spiral flow by convection caused by a circulating fan (sirocco fan) 104 provided above the heating-target-accommodating casing 102 to increase, at a high rate, the temperature of a material W to be heated. The heating furnace is arranged so as to form a circulating flow of the hot air such that the hot air is drawn into the heating-target-accommodating casing 102 through the bottom of the heating-target-accommodating casing 102 by the rotation of the circulating fan 104, passes through the heating-target-accommodating casing 102, and is blown out of the circulating fan 104 into a circulation path 103 between the heating-target-accommodating casing 102 and the furnace body 101 surrounding the heating-target-accommodating casing 102 to flow downward. A door 107 is provided at a second heating-target-transport opening 106 of the heating-target-accommodating casing 102. The circulation path for uniform circulation of the hot air through the entire circumferential region between the furnace body 101 and the heating-target-accommodating casing 102 is maintained by closing the door 107. The material W to be heated is moved into or out of the furnace by opening a door 109 at a first heating-target-transport opening 108 and the door 107 at the heating-target-accommodating casing 102 in the furnace body 101, and heat treatment is performed as a batch treatment.
As an ordinary continuous-type furnace, a long tunnel-type furnace not shown in the drawings exists in which a material to be heated carried into the furnace through a heating-target-carry-in opening at one end is heated to a predetermined temperature while being moved toward a heating-target-carry-out opening at the other end.
Since batch type treatment is carried out in the heating furnace shown in FIG. 9, there is a problem described below. Each time a material to be heated is carried into or out of the furnace, a large amount of in-furnace hot air flows out of the furnace and cold air outside the furnace flows into the furnace. The interior of the furnace is thereby cooled. Therefore, the thermal efficiency is low and the treatment time is long.
Also, since the circulating fan 104 used in this furnace is a sirocco fan constructed so that blades are exposed, there is a problem that in actuality the desired circulating flow is not generated and high-rate heating cannot be achieved. The amount of air caused by a sirocco fan to flow is determined by the design of a casing surrounding the sirocco fan. If the blades of a sirocco fan are exposed without being covered with a casing, the desired amount of flowing air cannot be obtained. Therefore, if only a sirocco fan having its blades exposed is provided, it is incapable of static-pressure recovery and only agitates air around the fan, resulting in a failure to generate a flow circulating through the entire furnace.
Even if a casing is provided to obtain the desired amount of flowing air, the circulating flow is generated as a spiral and is, therefore, formed in a one-sided condition and hot air cannot be brought into uniform contact with the material to be heated. Thus, there is a problem that heating unevenness occurs easily.
Moreover, in the case of heating by hot air circulating while forming a spiral, the interior of the furnace cannot be divided into a heating zone and a soaking zone. For this reason, it takes time to increase the temperature of the material to be heated to a predetermined point. The influence of this is considerable particularly in the case of heating of a material to be heated such as aluminum with which it is difficult to set a large thermal head. For example, annealing (solution annealing) of an aluminum alloy is performed at a temperature close to the melting point (softening point) of the aluminum and it is, therefore, impossible to reduce the temperature rise time (time required for reaching the solution annealing temperature) by setting a large thermal head because of the risk of solution damage to or deformation of the material to be heated. Thus, increasing the temperature of a material to be heated necessarily depends on heating by convection heat transfer in the case of a furnace in which heating by radiation heat transfer is limited due to the existence of a limit furnace temperature. In ordinary cases of T6 treatment in a medium temperature range of about 500° C., the thermal head is small and, therefore, the proportion of the amount of heating by convection heat transfer is increased while the proportion of the amount of heating by radiation heat transfer is reduced. The amount of heat transfer by convection heat transfer in the case of using a basket is about 85% and the amount of heat transfer by radiation heat transfer is about 15%. Since the heating power by convection heat transfer is determined by a function of the flow rate and the flow velocity of the heated fluid, it is very important to suitably design the circulating fan. In actual designing of the furnace, however, the flow rate or the flow velocity of the circulating fan cannot be increased without limitation and there is a limit to the increase in size of the fan to be installed in relation to the size of the furnace body. That is, it is difficult to improve the heating power by convection heat transfer if the furnace body is small.
Further, since a material to be heated is placed at a center of the furnace body 2 and since the circulating path is provided therearound, there is a problem that the amount of dead space is large; the treatable amount of material to be heated is reduced with respect to the furnace capacity; and the heating efficiency is low.
In the case of the tunnel-type continuous treatment furnace, there is a problem that the size of the furnace body is increased. In particular, in the case of heating of a material to be heated such as aluminum with which it is comparatively difficult to set the desired thermal head, the required heating time is long and there is a tendency toward a further increase in the length of the furnace.
On the other hand, the form of production has changed continuously and diversified and demands for various heating facilities and heat treatment facilities other than the existing demand for reducing the production cost by using a large continuous furnace have arisen in relation to the materials and forms of products, the amounts of production and so on. For example, it is desirable that a heat treatment furnace of a small amount of processing should be placed at an end of a casting line to enable a produced casting to be directly heat treated in the final step of the casting line, whereby the need for the wasteful method of temporarily cooling a casting and thereafter heating the casting from ordinary temperature is eliminated. Also, in production of an aluminum casting, there is a need to heat the materials one by one to perform primary heating, secondary heating, solution annealing and age-hardening. In such a case, it is desirable to provide a heat treatment furnace of a small amount of processing capable of carrying in, transporting and carrying out pieces of material to be heated one by one. The same can be said with respect to nonferrous metal alloys and steel as well as aluminum products. Such a demand cannot be easily met by using a conventional large tunnel-type continuous furnace presupposing large-amount treatment.
It is, therefore, an object of the present invention to provide a continuous-type hot-air circulation furnace small in size but having a large throughput. Another object of the present invention is to provide a hot-air circulation furnace capable of uniformly heating a material to be heated. Still another object of the present invention is to provide a hot-air circulation furnace capable of forming a heating zone and a soaking zone.